In recent years, with an increase in the amount of information processed in information equipment, audiovisual equipment or the like, attention has been directed to an information recording medium such as an optical disk allowing easy data access and capable of storing large volumes of data and responding to the miniaturization of equipment. Also, the higher-density recording of information has been studied. As an information recording medium capable of high-density recording, an information recording medium with respect to which information is recorded and/or reproduced using a recording/reproducing apparatus provided with an optical head including a laser light source with a wavelength of about 400 nm and a focusing lens with a numerical aperture (NA) of 0.85 has been suggested (see Patent document 1, for example). In this information recording medium, it is possible to store data with a capacity of about 25 GB in a single recording layer and about 50 GB in two recording layers, for example.
Now, the structure and manufacturing method of a conventional multilayer information recording medium described in Patent document 1 will be described with reference to FIG. 13A to FIG. 15J.
FIG. 13A to FIG. 13F show a method for manufacturing a substrate production die (stamper) used when producing the conventional multilayer information recording medium. First, a photosensitive material such as photoresist is applied onto a glass plate 201, thereby forming a photosensitive film 202 (see FIG. 13A). Then, using a laser beam 203, an exposure is performed for transferring a pattern of pits and guide grooves to the photosensitive film 202 (see FIG. 13B). In FIG. 13B, numeral 202adenotes a portion irradiated with the laser beam 203 (an exposed portion). The photosensitive material in the exposed portion undergoes a developing process so as to be removed, so that an optical recording master 205 in which a pattern 204 of pits and guide grooves is formed on the glass plate 201 is obtained (see FIG. 13C). Next, an electrically conductive film 206 is formed on the pattern 204 by sputtering, vapor deposition or the like. This transfers the shape of the pattern 204 onto the electrically conductive film 206 (see FIG. 13C and FIG. 13D). Subsequently, a plating film 207 is formed on the electrically conductive film 206, thereby increasing the rigidity and thickness of the electrically conductive film 206 (see FIG. 13E). Thereafter, a laminate of the plating film 207 and the electrically conductive film 206 is peeled off from the optical recording master 205, thus obtaining a stamper 208 (see FIG. 13F).
FIG. 14 is a sectional view showing the conventional multilayer information recording medium. This multilayer information recording medium includes a first signal substrate 301. A first information recording layer 302 is disposed on the first signal substrate 301, and a second signal substrate 303 is disposed on the first information recording layer 302. A second information recording layer 304, a transparent layer 305 and a transparent substrate 306 are disposed in this order on the second signal substrate 303. The transparent layer 305 is provided for attaching the transparent substrate 306 to the second information recording layer 304.
The first signal substrate 301 has a surface with pits and guide grooves serving as an uneven information surface. This information surface is formed when molding the first signal substrate 301 by an injection compression molding using the stamper 208 shown in FIG. 13F. The thickness of the first signal substrate 301 is about 1.1 mm. The first information recording layer 302 and the second information recording layer 304 each include a recording film, a reflecting film, etc., and are formed by sputtering, vapor deposition or the like.
The second signal substrate 303 is formed by attaching a signal transfer substrate having an uneven surface to a photocurable resin applied by spin-coating, curing the photocurable resin and then peeling off the signal transfer substrate from the photocurable resin. The signal transfer substrate has an uneven surface similarly to the stamper 208 shown in FIG. 13F.
The transparent substrate 306 is formed of a material that is adequately transparent to recording light and/or reproducing light. The transparent layer 305 is formed of a photocurable resin and an adhesive such as a pressure-sensitive adhesive. The average thickness of the combination of the transparent substrate 306 and the transparent layer 305 is about 0.075 mm. With respect to such a multilayer information recording medium, information is recorded/reproduced by allowing a recording/reproducing laser beam to enter from the side of the transparent substrate 306.
The following is a more detailed description of the method for manufacturing the conventional multilayer information recording medium with reference to FIG. 15A to FIG. 15J.
First, a first information recording layer 402 is formed on an information surface of a first signal substrate 401 by sputtering, vapor deposition or the like. The first signal substrate 401 is kept fixed to a rotation table 403 by means of a suction device or the like (see FIG. 15A). Next, onto the first information recording layer 402, a coating 404 containing a photocurable resin is applied in such a manner as to form a circle with a desired radius using a dispenser (see FIG. 15B). Then, the rotation table 403 is rotated, thereby spreading the coating 404. At the time of spreading, any excess resin and air bubbles are removed by centrifugal force. The spread coating 404 can be controlled to have a desired thickness by setting the viscosity of the coating 404, the rate of revolutions of the rotation table, the period for rotating the same and the atmospheric conditions (temperature, humidity etc.) as needed. After the rotation, the coating 404 is cured by light irradiation using a light irradiator 405, thus obtaining a photocurable resin layer 404′ (see FIG. 15C).
On the other hand, a signal transfer substrate 406 is fixed onto a rotation table 407. The signal transfer substrate 406 has an uneven surface similar to the stamper 208 shown in FIG. 13F (see FIG. 15D). Onto the signal transfer substrate 406, a coating 408 containing a photocurable resin is applied in such a manner as to form a circle with a desired radius using a dispenser. Then, the rotation table 407 is rotated, thereby spreading the coating 408. The thickness of the spread coating 408 can be controlled similarly to the case of the coating 404 (see FIG. 15E). After the rotation table 407 is stopped, the coating 408 is cured by light irradiation using a light irradiator 409, thus obtaining a photocurable resin layer 408′ (see FIG. 15F).
Subsequently, on the rotation table 403, a substrate 410 and a substrate 411 are stacked via a coating 412 containing a photocurable resin such that the photocurable resin layers 408′ and 404′ face each other. In this state, the rotation table 403 is rotated (see FIG. 15G). By the rotation of the rotation table 403, the coating 412 is controlled (spread) to have a desired thickness. Thereafter, the coating 412 is cured by light irradiation using the light irradiator 405, thus obtaining a photocurable resin layer 412′ (see FIG. 1511). Then, the signal transfer substrate 406 is peeled off from the photocurable resin layer 408′.
It should be noted that the photocurable resin contained in the coating 404 is selected from resins having an excellent adhesiveness to the first information recording layer 402 and the photocurable resin layer 412′. The photocurable resin contained in the coating 408 is selected from resins having an excellent peelability from the signal transfer substrate 406 and an excellent adhesiveness to the photocurable resin layer 412′ (see FIG. 15H). The viscosities of the coatings 404, 412 and 408 are all adjusted to be about 150 mPa·s so that a thin photocurable resin layer can be formed. Incidentally, an integral body of the photocurable resin layers 404′, 408′ and 412′ (also referred to as a resin layer) corresponds to the second signal substrate 303 in FIG. 14. For convenience of description, the above-noted integral body is illustrated to be thicker than the second signal substrate 303 in FIG. 14.
Next, a second information recording layer 413 is formed on a surface of the photocurable resin layer 408′ opposite to the side of the first signal substrate 401, namely, a second information surface by sputtering, vapor deposition or the like. On the second information recording layer 413, a coating containing a photocurable resin is applied for forming a transparent layer 415. Then, after a transparent substrate 414 is attached to the applied coating, the rotation table 403 is rotated, thereby removing air bubbles mixed into the coating and spreading the coating. Thereafter, the coating is irradiated with light having a desired wavelength through the transparent substrate 414, thus curing the photocurable resin. Thus, the coating is formed into the transparent layer 415 (see FIG. 15I).
Patent document 1: JP 2002-092969 A